1. Field of the Invention
A certain aspect of the present invention relates to mobile communication technologies. More particularly, the present invention relates to a mobile terminal and a method used in the mobile terminal.
2. Description of the Related Art
A mobile terminal in the standby mode performs a cell search and selects a suitable cell based on the results of the cell search. Cell selection is performed regardless of whether the mobile terminal is stationary or moving. When a mobile terminal in the standby mode is stationary, substantial changes in the radio propagation conditions are not normally observed and therefore it is not normally necessary to frequently perform cell selection. Still, however, instantaneous measurements of radio propagation conditions change continuously. Therefore, if cell selection is performed based on instantaneous values of radio propagation conditions, unnecessary reselection of cells is performed frequently and as a result, efficiency of the network or the mobile terminal is reduced. In a related-art method, the mobile terminal averages measurements of radio propagation conditions to prevent such frequent reselection of cells and thereby to lengthen the standby time. In the method, the mobile terminal averages the measurements based on an averaging (filtering) factor that is a fixed value provided in the mobile terminal.
Meanwhile, Japanese Patent Application Publication No. 2007-258845 and Japanese Patent Application Publication No. 2007-202096 disclose a method for determining whether to switch the mobile terminal to an intermittent reception mode or a different frequency measurement mode based on an average of signal measurements.
A mobile terminal regularly receives broadcast information from a base station which includes parameters related to a cell measurement interval and cell reselection specified by the network. Such parameters may change according to the conditions of cells. If the conditions of cells change, the timing and frequency at which cell reselection is required may also change and therefore it is preferable to change the method of averaging measurements of, for example, the signal reception level. Particularly, when the mobile terminal is moving, suitable cells change constantly and therefore it is preferable to frequently calculate or update the average of reception level measurements.
However, in the related art, the averaging factor provided in the mobile terminal is fixed (constant) and the same averaging method is used regardless of the cell conditions. Accordingly, with the related-art technologies, it is difficult to appropriately average measurements of communication quality of neighboring cells according to actual cell conditions and thereby to select a suitable cell.
To summarize, related-art technologies have disadvantages as described below.
(a) The averaging factor used to average measurements of cell quality for cell selection is not changed even when parameters related to a measurement interval and cell selection specified by the network are changed. Therefore, it is difficult to average the measurements according to actual or current conditions of cells.
(b) Although communication conditions are much influenced by the moving speed of the mobile terminal, the averaging factor used in averaging measurements of cell quality is not changed even when the moving speed changes. Therefore, it is difficult to average the measurements according to the communication conditions.